As is well known, an alternator is an electrical machine in which an alternating current is generated in its stator windings under the effect of inductor or rotor winding, through which an excitation current flows when the rotor winding is put into rotation. At the output of the stator windings, a diode bridge rectifies the alternating current so as to deliver a uni-directional, or direct current, voltage to the battery of the vehicle.
The rectified voltage has to be regulated in such a way as to remain constant regardless of the speed of rotation of the alternator, and regardless of the load on the battery. The output voltage of the alternator thus has a regulation waveform which reflects the excitation current, this waveform having a period T. The period T comprises a first phase T.sub.1 and a second phase T.sub.2 alternating with each other, i.e. having opposite signs whereby the amplitude increases in one phase and decreases in the other. In the first phase, the battery voltage is applied to the rotor winding, which enables the current flowing in the rotor winding to increase; while in the second phase, the battery voltage is not applied to the rotor winding, and this reduces the current in the rotor winding. The ratio T.sub.1 /(T.sub.1 +T.sub.2) is called the cyclic ratio.
It has previously been proposed to carry out the regulation of the rectified voltage output from the alternator using digital techniques. For example, European patent specification No. EP 0 481 862 describes a procedure for regulation by digital processing, in which the output voltage of the alternator is measured at each period T by sampling. This sampling comprises the step of detecting, in each period T, whichever is the longer one of the phases T.sub.1 or T.sub.2. The sampling step is performed over a time period T.sub.e which occurs in the middle of the longer one of the two phases. The numerical value of the measured voltage is then compared with a reference value of the voltage, and from this comparison the value of the phases T.sub.1 and T.sub.2 in the next following period is deduced.
That method of regulation, although satisfactory because it enables variations due to the waveform to be minimised, by centring the measurement of the voltage to be regulated on the longer of the two phases, is relatively complicated to perform in practice. In this connection, it is necessary, firstly to find out which one of the two phases is longer than the other, and secondly to centre the sampling period in the middle of that phase. It has also been found that regulation errors tend to appear when a cyclic ratio diminishes from a value greater than 50% (where the first phase T.sub.1 is the longer one) to a value less than 50%, in which it is the second phase T.sub.2 that is the longer one.